1. Field of the Invention
This invention relates to improved apparatus for analysis of compounds by liquid chromatography/mass spectrometry. More particularly, this invention relates to apparatus which will provide a stream of electrons flowing in the ionization chamber of the mass spectrometer in the same direction as the spray of liquid/vapor from the liquid chromatograph and at an energy level sufficient to: (1) bombard gaseous molecules which form ions and fragment ions; and (2) form hydronium ions which act as reagent ions for further ionization of the molecules in the spray.
2. Description of the Related Art
The analysis of molecules of decomposable liquids or solids, using liquid chromatographic-mass spectrometry apparatus is known wherein the liquid eluent containing the molecule to be analyzed is sprayed into an ionization chamber, juxtapositioned between the chromatographic column and the mass spectrometer, and formed therein into an ionized adduct by ionization of the solvent in the spray through exposure to a beam of high energy electrons also introduced into the ionization chamber. Typical of such apparatus is the liquid chromatography mass spectrometry system disclosed in McLafferty et al U.S. Pat. No. 3,997,298.
Another system used in liquid chromatography-mass spectrometry analysis of decomposable molecules is the thermospray method wherein an ionized solvent, e.g., ammonium acetate, is used with the liquid/solid compound to be analyzed. The liquid eluent from the chromatograph column containing such an ionized solvent is introduced into the ionization chamber through one or more heated nozzles. The ions already present in the solution evaporate in solvated form from droplets in the thermally produced spray. For example, if the solution being analyzed comprises a compound dissolved in the aforementioned ammonium acetate solvent, the ammonium and acetate ions formed in the spray may form ionized adducts with the compound which ionized adducts may then be analyzed in the mass spectrometer. This thermospray form of ionization of liquids is described by M. L. Vestal in the International Journal of Mass Spectrometry and Ion Physics, 1983, Vol. 46, at pp. 193-196.
In such an apparatus, the thermospray is introduced through one or more heated nozzles into one end of an ionization chamber having an exit port to a vacuum pump at the opposite end whereby the solvent-spray will flow toward the exit port of the ionization chamber. At a point in the sidewall of the ionization chamber, spaced from the entrance nozzle and the exit port, is an opening, which may comprise a cone depending from the sidewall into the ionization chamber with an opening at the apex of the cone. The ionized adducts pass through this opening into the mass spectrometer which is maintained at a higher vacuum (lower pressure).
Flow of the ionized adducts through the opening in the sidewall of the ionization chamber into the mass spectrometer involves a change in direction of the flow of the ionized adducts influenced by the higher vacuum at the mass spectrometer entrance on these ionized adducts. Thus some fraction of the material to be analyzed flows into the mass spectrometer while most of the vaporized solvent passes on to the exit port at the end of the ionization chamber to the vacuum pump connected thereto.
To assist in this separation of the ions - or ionized adducts - to be analyzed from the remainder of the solvent, it has been proposed by Lewis et al, in U.S. Pat. No. 4,647,772, to locate a "repeller" electrode in the ionization chamber downstream of the opening in the sidewall to the mass spectrometer. This "repeller" electrode is maintained at a potential sufficient to provide a repulsive electrostatic field in the ionization chamber which will retard the flow of ionized adducts in the chamber and deflect the flow to the opening in the chamber into the mass spectrometer.
While such modifications have improved the analysis of decomposable liquids and solvents by the liquid chromatography/mass spectrometry method, the method, in the main, is still limited to ascertaining the molecular weight of the unknown compound and using this to identify the compound. It would, therefore, be desirable to be able to fragment the unknown molecule, as in gas chromatography/mass spectrometry systems, to provide further information as to the structure of the unknown molecule by identification of peaks associated with fragmented groups of the molecule.